Yes, this is established in authentic hadith. The Prophet (peace be upon him) said of Surah al-Ikhlas ("Qul Huwa Allahu Ahad"), "By Him in whose hand is my soul, it is equal to one third of the Quran." Scholars explain that the Quran's message revolves around three themes — tawhid, rulings, and reports of the unseen — and this surah is a pure and complete statement of tawhid (Allah's oneness), so it equals a third in reward and meaning, though reciting it does not literally substitute for reciting the whole Quran.
Evidence — the daleel
Quran 112Sahih al-Bukhari 5013 Ṣaḥīḥ
On deen2u
Informational, not a personal fatwa. Consult a qualified scholar for rulings on your situation.